(AI) Braille: read a lone key signature in a keyboard segment heading

[wolfgang-aura]

Fixes #1986: keyboard braille no longer aborts with AttributeError: 'KeySignature' object has no attribute 'symbol' when a segment opens with a key change and no meter change.

BrailleGrandSegment.extractHeading tested isinstance on the grouping rather than its element, so a grouping holding only a KeySignature was passed on as the time signature. As you suggested in the issue, the SIGNATURE branch now walks the grouping and takes the first KeySignature and the first TimeSignature, without assuming their order or count.

test_keyboardInteriorKeyChangeWithoutMeterChange in braille/test.py builds an 8-measure keyboard score with a BrailleSegmentDivision and a change to two sharps at measure 5, and checks the exact braille. It fails at master with the error above and passes with this change.

How this was tested

pytest music21/braille passes (73 tests), and python -m music21.braille.<module> passes for basic, segment, translate, test, noteGrouping, text, objects and lookup, which also runs their doctests. Windows, Python 3.14 only.

I left the handleArticulations fingering warning path that the issue also mentions for a separate change.

🤖 Generated with Claude Code

[coveralls]

coverage: 93.381% (+0.002%) from 93.379% — wolfgang-aura:fix-1986-braille-keysig into cuthbertLab:master

[mscuthbert]

LGTM -- Thank you!